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In the present study, PHA production from sheep cheese whey (SCW) through a 3-stage process (dark fermentation, PHA-biomass selection and PHA accumulation process) was investigated. To this aim, batch dark fermentation (DF) tests were performed on raw SCW without any addition of biomass inoculum nor any pre-treatment of the substrate and adopting different pH operative conditions. The DF effluents, rich in organic acids, were characterized and used for selection and enrichment of a PHA-storing microorganisms and in PHA accumulation tests. The impact of adopting different operating pH in the DF tests was evaluated in terms of organic acids and biohydrogen yields, PHA composition and yields in order to evaluate the feasibility of the waste biorefinery process here proposed. Results showed that the production of PHA from SCW is a promising valorisation approach. Operating pH adopted during the fermentation step affected the biohydrogen production yield as well as the organic acid type and, in turn, the yield and the composition of the obtained biopolymer. An overall yield of 5.3 L H2 and 16.1 g PHA per litre of SCW could be obtained by setting the fermentation operative pH = 6.